The invention relates to apparatus for the conversion of alternating current (AC) to direct current (DC) under low voltage and high power rating as applicable for electrolysis DC power supply, or to aluminum melting, for instance. The technique used generally combines a transformer and a rectifying circuit connected to the secondary of the transformer. Low voltage and high currents impose limitations in the design of the basic transformer, in particular relatively few secondary turns are used to provide a low voltage and multiple windings are required which are associated with an array of rectifier devices in order to reduce ripple. More particularly, with a threephase AC power supply, it is customary to look for a 12-pulse rectifying system.
A perfect 12-pulse rectifying system does not associate naturally with multiple secondary windings of a transformer. The prior art has not been able to achieve such a system with good symmetry because a distributed vectorial representation of the twelve AC phases at the input of the rectifying circuit has been obtained by a transformer construction in which the required turns ratios are hard to implement and leakage reactances are difficult to match. In addition, the required reduction of harmonics necessitates the use of interphase reactors.
An object of the present invention is to realize a nearly ideal multi-winding transformer-rectifier apparatus for low voltage and high current.